A Modified Disease Risk Index for Hop Powdery Mildew

The hop powdery mildew fungus, Podosphaera macularis, has overcome host resistance when cultivars with a given form of genetic resistance are broadly deployed, as most recently experienced in the partially resistant hop cultivar Cascade. Hop is a long-lived perennial grown under production contracts; therefore, growers of Cascade hops must now manage powdery mildew because of contractual obligations. We developed and validated a modified powdery mildew risk index to aid in matching fungicide application intervals for foliar powdery mildew to weather favorability specifically for this cultivar. The modified risk index preserves the form and logic of the original hop powdery mildew risk index but lowers the cardinal temperature from 30 to 28°C and introduces a low-temperature rule that reduces estimated daily risk points when temperatures are ≤4°C for ≥2 h. During eight location-years of validation in commercial yards in Washington State, the modified disease risk index resulted in disease control comparable to fungicide applications made at regular intervals yet with up to two fewer applications in years predicted to be less favorable to powdery mildew. We separately evaluated the benefit of fungicide applications made after bloom for managing the cone phase of powdery mildew. In 5 years of studies, fungicide applications ceasing in mid-July yielded disease control and hop brewing quality metrics similar to applications made into mid-August. An effective and minimal input program for the cultivar Cascade can be achieved by adjusting fungicide application intervals appropriately in spring and omitting unnecessary fungicide applications 4 to 6 weeks before harvest. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2025.